McIntyre & Moore Booksellers, whose wunderkammer of used books was recently exiled from Davis Square (and long ago displaced from Harvard Square), reopened today in Porter Square, at 1971 Mass. Ave., Cambridge. (They're in good company here: Bob Slate—purveyor of fountain pens and rampant Moleskinery—and Stellabella Toys are right upstairs.) They describe their stock as "uncommon titles in interesting subject areas, usually in fields in which it's very hard to be gainfully employed"—or, in a......